1. Field of the Invention
The invention concerns a method for setting the speed of a relative movement between a print head and a recording medium during a printing process in a thermotransfer printing device, in particular for setting the transport speed of a mail piece in a thermotransfer printing franking machine. The invention also concerns a thermotransfer printing device.
2. Description of the Prior Art
Thermotransfer printing technology has been known for some years and has proven to be of value in the field of label and ticket printing as well as in franking of mail using EDP (electronic data processing).
Configurations known as 2D barcodes (data matrix symbol) which, among other things, include data regarding the shipped product, the postage account, and the recipient address, are in particular used in the latter cited application. The 2D barcode itself is executed as a printed, rectangular region that is subdivided into rectangular (mostly square) fields (known as modules). This region includes solid lines and lines with light and dark modules in alternation for recognition and classification of the symbol. The usable information is located between the recognition lines in the form of light or dark modules that are composed of a number of pixels (for example 6×6 pixels). The 2D barcode additionally includes redundant data, such that up to 25% of the errors in the individual modules can be automatically corrected by suitable correction algorithms when, for example, parts of the 2D barcode have been covered over or become unreadable.
In the franking of mail by a thermotransfer printing, an ink ribbon is located between the letter and a print head that has a large number of heating elements arranged in a column perpendicular to the letter travel direction. When a heating element is activated and heated, the ink layer in the ink ribbon melts and is transferred to the paper surface of the letter. In order to enable a two-dimensional printing of the letter, the ink ribbon moves below the print head together with the letter. The transport speed is determined by an encoder, with the encoder wheel resting on the ink ribbon, for example.
The machine readability, especially of 2D barcodes with very small module sizes (for example 0.5 mm×0.5 mm) decisively depends on the print quality of the thermotransfer printing device. Significant factors for the print quality are a uniform size of the modules both in the transport direction and in the print column direction as well as the uniform areal coverage across the 2D barcode. Fluctuations in the transport speed can cause both compressions and expansions in the printout of the data matrix symbol that are known as “ripples”. The transport speed is influenced by production tolerances of the thermotransfer printing device (for example a franking machine) that can change during the lifespan of the device. For a thermotransfer printing device fashioned as a franking machine, the transport speed also depends on the thickness of the letter to be franked. Given the same drive control, thick and heavy letters are thus transported slower (due to inertia or as a consequence of higher friction) than thin, light letters.